Engine systems may employ humidity sensors to monitor the operating conditions of the engine. Humidity sensors located within the intake manifold can factor into a determination of an air-to-fuel ratio, meter an amount of exhaust gas recirculation in an intake, and others.
Humidity sensors may measure relative humidity, absolute humidity, or specific humidity.
One example humidity sensor degradation diagnostic method is disclosed in U.S. Pat. No. 6,073,480 wherein fault within a capacitance humidity sensor is determined by using a condensation heater coupled to the humidity sensor to increase the local temperature and thus the corresponding humidity. Degradation is then determined by comparing the capacitance of the sensor before and after the heater is activated. Specifically, U.S. Pat. No. 6,073,480 determines capacitance humidity sensor degradation by cycling between temperatures above and below a dew point and compares the change in relative humidity to the change in capacitance. If the capacitance does not change an expected amount between an above-dew-point and a below-dew-point temperature, sensor degradation is indicated.
However, the inventors herein have recognized some potential issues with the above approach. Namely, the degradation indication relies on a properly functioning heater and temperature sensor. Heater or temperature sensor degradation may therefore be misdiagnosed as sensor capacitor degradation, or vice versa. The above method also relies on a modeled local temperature in response to the activation of a heater; however the temperature near the humidity sensor is not isolated and may be affected by atmospheric temperature or the heat of the engine, leading to inaccurate results.
The inventors recognized that the above issues may be addressed specifically in systems with a condensation-heater and temperature sensor coupled to the humidity sensor. Embodiments of these systems use the absolute humidity detected, a measured temperature, and an assumed or measured total pressure to determine other measures of humidity. By a similar method, absolute humidity can be found to be a function of temperature, total pressure, and specific humidity.
For example, specific humidity (also known as humidity ratio) is not affected by ambient temperature or pressure, because it is a mass ratio of water to dry air. Therefore, the relative humidity for a given absolute humidity can be modeled as a function of pressure and temperature. The inventors found that by comparing the modeled output of the humidity sensor to the actual output of the humidity sensor, degradation can be determined if the difference between the modeled output and actual output surpasses a margin of error threshold.
In other embodiments using particular humidity sensors, the relative humidity may be calculated as a function of the specific humidity and the measured temperature. Humidity sensor degradation may then be indicated if the calculated relative humidity exceeds the known limits for relative humidity.
In one embodiment, a method, comprises operating a sensor in an engine intake, a housing of the sensor including a temperature-sensing element, a heater, and a capacitance-sensing element; and individually distinguishing between each of heater, temperature-sensing element, and capacitance-sensing element degradation. In this way, it is possible to provide accurate diagnostics and prognostics.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.